In the recent years, magnetic cards enjoy widespread use in a variety of fields. In particular, the magnetic cards find an expanding application as rewritable or prepaid cards in which monetary information is recorded as magnetic bits of information so that the amount of money may be reduced on every use.
In this application, the safety of the card system is fatally lost if magnetic cards are not fully protected against forgery by the alteration of recorded data or counterfeit cards can be easily produced. There is a strong demand for magnetic cards having a protective function of preventing the alteration of recorded data. To meet the demand, a variety of magnetic cards have been proposed and used in practice. For example, if magnetic cards are locally provided with a region of a special material, they are difficult to counterfeit and whether they are false or true can be judged by detecting the special region. Magnetic cards with a complex layer arrangement are also known.
These magnetic cards having the protective function incorporated therein are difficult to forge or duplicate a number of counterfeit cards. However, there is still a possibility that the monetary information in an exhausted card be restored to the initial information by false alteration, for example, by rewriting the monetary information. One countermeasure is to punch holes in the card in accordance with the number of uses although this yet raises several problems that precise agreement is not expectable, punched chips are left, and repair by refilling the holes is possible. Another possible countermeasure is to record visible data corresponding to the number of uses by thermographic recording. These cards carrying visible data, however, are weak against staining because the visible data must be optically read. The falsification of the record is easy on account of visible data. Another problem is that optical readers are generally expensive. There are available no practical means which are quite effective for protection against forgery.